1. Field of the Invention:
The present invention relates to non-magnetic steel materials suitable for use in various steel and concrete structures, such as magnetic floating high-speed rail-ways, nuclear fusion facilities and marine structures and appliances where a non-magnetic property is required.
The steel materials suitable for the above applications must have good corrosion resistance, and therefore the present invention particularly concerns non-magnetic steel compositions useful for preventing the decay of marine steel and concrete structures and similar structures which may be built on seashores.
In recent years, various preventive methods for preventing the decay of steel and concrete structures which are built on the ocean and seashores have been proposed, and indeed some of these have already been put into practice.
The principal causes for the decay of steel structures include the corrosion by the seawater itself and corrosion by the sea salt particles. Meanwhile, the principal cause for the decay of concrete structures has been found to be attributable to the fact that reinforcing steel bars or wires embedded in the concrete structure are corroded by salts contained in sea sand used when mixing the concrete, or by sea salt particles which permeate into a concrete structure built on a seashore or in seawater. The corrosive salts have an increased volume of about 2.2 times the iron, and the concrete fails to withstand the expansion forces of the corroding steel bars or wires. The concrete thus cracks along the embedded reinforcing bars or wires. When the cracks grow to about 0.2 mm or larger, external corrosive media, such as oxygen, salts, and carbon dioxide in the air, penetrate these cracks to reach the interior of the concrete mass where the reinforcing bars or wires are embedded. This further promotes the corrosion of the bars or wires, or accelerates neutralization of the concrete, causing premature decay of the concrete structures.
2. Description of the Related Art:
For the purpose of preventing such decay of concrete structures, the present inventors have conducted extensive studies and experiments to improve the salt resistance of the reinforcing steel bars or wires, by controlling their chemical composition. As a result, concrete reinforcing steel bars or wires have been developed which have significantly improved salt resistance, as disclosed in Japanese Laid-Open Patent Application Nos. Sho 57-48054 and Sho 59-44457, and as widely published in "Offshore Goteborg '81", Paper No. 42, Goteborg Sweden, 1981; "Cement Concrete" No. 434 (1983), Pages 23 to 31; "Corrosion of Reinforcement in Concrete Construction" page 419, 1983; and "Kenchiku No Gijutsu Seko" (Practice for Building Construction) No. 229, 1985, Jan. Pages 155 to 164, published by Shokokusha, Japan.
Also, details of the salt resistance mechanism at the initial stages of steel compositions for reinforcing bars or wires and which contribute to improving the salt resistance of reinforcing bars or wires per se are reported in these technical papers.
Also, in recent years, trials have been made at preparing steel materials containing 15% or more manganese for the purpose of obtaining non-magnetic properties, but one critical problem confronted by all of these Mn-containing steel materials is that the rust generation rate is remarkable, and higher than ordinary carbon steel; hence a higher corrosion rate is experienced with the presence of a very small amount of salt.